Surface acoustic wave (SAW) devices use one or more interdigitated transducers (IDTs), and perhaps reflectors, provided on a piezoelectric substrate to convert acoustic waves to electrical signals and vice versa. SAW devices are often used in filtering applications for high-frequency signals. Of particular benefit is the ability to create low loss high order bandpass and notch filters without employing complex electrical filter circuits, which may require numerous active and passive components. A common location for a filtering application is in the transceiver circuitry of wireless communication devices.
With reference to FIG. 1, a typical SAW device 10 on a temperature compensated bonded substrate is illustrated. The SAW device 10 will generally only include a piezoelectric substrate 12, which has a surface on which various types of SAW elements, such as IDTs and reflectors, may be formed. In a temperature compensated bonded substrate, the piezoelectric substrate 12 resides on a supporting substrate 14 as shown in FIG. 1. The mechanical and thermal properties of the supporting substrate 14 and the piezoelectric substrate 12 act in conjunction to render the SAW device 10 more stable to temperature variations. As illustrated in this example, a dual-mode SAW (DMS) device is provided, wherein at least two IDTs 16 are placed between two reflectors 18. Both the IDTs 16 and the reflectors 18 include a number of fingers 20 that are connected to opposing bus bars 22. For the reflectors 18, all of the fingers 20 connect to each bus bar 22. For the IDTs 16, alternating fingers 20 are connected to different bus bars 22, as depicted. Notably, the reflectors 18 and IDTs 16 generally have a much larger number of fingers 20 than depicted. The number of actual fingers 20 has been significantly reduced in the drawing figures in an effort to more clearly depict the overall concepts employed in available SAW devices 10 as well as the concepts provided by the present invention.
Notably, the fingers 20 are parallel to one another and aligned within an acoustic cavity, which essentially encompasses the area in which the reflectors 18 and the IDTs 16 reside. In this acoustic cavity, the standing wave or waves generated when the IDTs 16 are excited with electrical signals essentially reside within the acoustic cavity. As such, the acoustic wave energy essentially runs perpendicular across the various fingers 20. In the example illustrated in FIG. 1, one IDT 16 may act as an input while the other IDT 16 may act as an output for electrical signals. Notably, the IDTs 16 and the reflectors 18 are oriented in acoustic series, such that the acoustic wave energy moves along the cavity and perpendicularly across the respective fingers 20 of the IDTs 16 and the reflectors 18.
One issue with the SAW device 10 of FIG. 1 is that the conventional manufacturing process for such a device is expensive. More specifically, as illustrated in FIG. 2, the conventional manufacturing process begins by polishing surfaces of the piezoelectric substrate 12 and the supporting substrate 14. The polished surfaces of the piezoelectric substrate 12 and the supporting substrate 14 are then attached via a direct bonding process or some other similar bonding process. Then, as illustrated in FIG. 3, the piezoelectric substrate 12 is grinded to a desired thickness and polished. While not shown, SAW device elements such as, for example, IDTs 16 and reflectors 18, are then formed on the piezoelectric substrate 12. Because the conventional manufacturing process requires two polishing steps, the process is expensive. Thus, there is a need for an improved process for manufacturing SAW device substrates that eliminates the need for multiple polishing steps. Additionally, the challenges to direct bonding of highly dissimilar materials are sometimes insurmountable and the processes are often expensive.